A major histopathological hallmark of Alzheimer's Diseae (AD) is the presence of amyloid deposits within neuritic and difuse plaques in the parenchyma of the amygdala, hippocampus and neocortex (Glenner and Wong, 1984; Masters et al., 1985; Sisodia and Price, 1995). Amyloid is a generic tern that describes fibrillar aggregates that have a common β-pleated structure. These aggregates exhibit birefringent properties in He presence of Congo red and polarized light (Glenner and Wong, 1984). The diffuse plaque is thought to be relatively benign in contrast to the neuritic plaque which appears to be strongly correlated with reactive and degenerative processes (Dickson et al., 1988; Tagliavini et al., 1988; Yamaguchi et al., 1989; Yamaguchi et al., 1992). One of the principal components of neuritic plaques is a 42 amino acid residue amyloid-β (Aβ) peptide (Miller et al., 1993; Roher et al., 1993) that is derived from the much larger β-amyloid precursor protein, β APP (or APP) (Kang et al., 1987). Aβ 1-42 is produced less abundantly than the 1-40 Aβ peptide (Haass et al., 1992; Seubert et al., 1992), but the preferential deposition of Aβ 1-42 results from the fact that this COOH-extended form is more insoluble than 1-40 A β and is more prone to aggregate and form anti-parallel β-pleated sheets (Joachim et al., 1989; Halverson et al., 1990; Barrow et al., 1992; Burdick et al., 1992; Fabian et al., 1994). Aβ 1-42 can seed the aggregation of Aβ 1-40 (Jarrett and Lansbury 1993).
The APP gene was sequenced and found to be encoded on chromosome 21 (Kang al., 1987). Expression of the APP gene generates several Aβ-containing isoforms of 695, 751 and 770 amino acids, with the latter two APPs containing a domain that shares structural and functional homologies with Kunitz serene protease inhibitors (Kang et al., 1987; Kitaguchi et al., 1988; Polite et al., 1988; Tanzi et al., 1988; Konig et al., 1992). The functions of APP in the nervous system remain to be defined, although there is increasing evidence that APP has a role in mediating adhesion and growth of neurons (Schubert et al., 1989; Saitoh et al., 1994; Roch, 1995) and possibly in a G protein-linked signal transduction pathway (Nishimoto et al., 1993). In cultured cells, APPs mature through the constitutive secretory pathway (Weidemann et al., 1989; Haass et al., 1992; Sisodia 1992) and some cell-surface-bound APPs are cleaved within the Aβ domain by an enzyme, designated α-secretase, (Esch et al., 1990), an event that precludes Aβ amyloidogenesis. Several studies have delineated two additional pathways of APP processing that are both amyloidogenic: first an endosomal/lysosomal pathway generates a complex set of APP-related membrane-bound fragments, some of which contain the entire Aβ sequence (Haass et al., 1992; Golde et al., 1992); and second, by mechanisms that are not fully understood, Aβ 1-40 is secreted into the conditioned medium and is present in cerebrospinal fluid in vivo (Haass et al., 1992; Seubert et al., 1992; Shoji et al., 1992; Busciglio et al., 1993). Lysosomal degradation is no longer thought to contribute significantly to the production of A β (Sisodia and Price 1995). The proteolytic enzymes responsible for the cleavages at the NH2 and COOH termini of A β are termed β (BACE) and γ secretase, respectively. Until recently, it was generally believed that Aβ is generated by aberrant metabolism of the precursor. The presence, however, of Aβ in conditioned medium of a wide variety of cells in culture and in human cerebrospinal fluid suggest that Aβ is produced as a normal function of cells.
The main focus of researchers and the principal aim of those associated with drug development for AD is to reduce the amount of Aβ deposits in the central nervous system (CNS). These activities fall into several general areas: factors affecting the production of Aβ, the clearance of Aβ, and preventing the formation of insoluble Aβ fibrils. Another therapeutic goal is to reduce inflammatory responses evoked by Aβ neurotoxicity.
Given that neurotoxicity appears to be associated with β-pleated aggregates of Aβ, one therapeutic approach is to inhibit or retard Aβ-aggregation. The advantage of this approach is that the intracellular mechanisms triggering the overproduction of Aβ or the effects induced intracellularly by Aβ need not be well understood. Various agents that bind to Aβ are capable of inhibiting Aβ neurotoxicity in vitro, for example, the Aβ-binding dye, Congo Red, completely inhibits Aβ-induced toxicity in cultured neurons (Yankner et al., 1995). Similarly, the antibiotic rifampacin also prevents Aβ aggregation and subsequent neurotoxicity (Tomiyama et al., 1994). Other compounds are under development as inhibitors of this process either by binding Aβ directly, e.g., hexadecyl-N-methylpiperidinium HMP) bromide (Wood et al., 1996), or by preventing the interaction of Aβ with other molecules that contribute to the formation of Aβ deposition. An example of such a molecule is heparan sulfate or the heparan sulfate proteoglycan, perlecan, which has been identified in all amyloids and is implicated in the earliest stages of inflammation associated amyloid induction.
Heparan sulfate interacts with the Aβ peptide and imparts characteristic secondary and tertiary amyloid structural features. Recently, small molecule anionic sulfates have been shown to interfere with this reaction to prevent or arrest amyloidogenesis (Kisilevsky, 199S), although it is not evident whether these compounds will enter the CNS. A peptide based on the sequence of the perlecan-binding domain appears to inhibit the interaction between Aβ and perlecan, and the ability of Aβ-derived peptides to inhibit self-polymerization is being explored as a potential lead to developing therapeutic treatments for AD. The effectiveness of these compounds in vivo, however, is likely to be modest for a number of reasons, most notably the need for chronic penetration of the blood brain barrier.
An alternative to a peptide-based approach is to elucidate the cellular mechanism of Aβ neurotoxicity and develop therapeutics aimed at those cellular targets. The focus has been on controlling calcium dysfunction of free radical mediated neuronal damage. It has been postulated that Aβ binds to RAGE (the receptor for advanced glycation end-products) on the cell surface, thereby triggering reactions that could generate cytotoxic oxidizing stimuli (Yan et al., 1996). Blocking access of Aβ to the cell surface biding site(s) might retard progression of neuronal damage in AD. To date there are no specific pharmacological agents for blocking Aβ-induced neurotoxicity.